There is great public and private interest in the use of employee wellness programs to improve health and lower health care costs, with such programs promoted in the Affordable Care Act and adopted by half of large firms. However, little rigorous evidence exists on the effects of such programs. Using a randomized controlled design, we propose to evaluate a year-long wellness intervention at BJ's Wholesale Club, a large multi-state U.S. business with 201 sites (clubs) and over 25,000 employees. The wellness intervention will consist of several components, including team-based wellness challenges, nutrition counseling, stress reduction, and physical activity. The intervention has been designed with and will be fielded through Wellness Workdays, an experienced and large-scale vendor. Randomizing this intervention across a set of treatment clubs and control clubs, we will evaluate its impact on 5 categories of outcomes gathered from both primary and administrative data sources: (1) self-reported health and well-being, (2) biometrics (such as body mass index, blood pressure, and blood sugar), (3) worker absenteeism and turnover, (4) health care spending and utilization, and (5) return on investment. Better information about the effectiveness of such programs should inform both employer investment and public policy.